1. Field of the Invention
The present invention relates to a method of making thermal printheads. More specifically, the present invention relates to a method of making thermal printheads of the type which comprise a glaze corner for carrying an array of heating dots therealong.
2. Description of the Prior Art
A thermal printhead of the above-described type is disclosed in Japanese Patent Application Laid-open No. 4(1992)-244861 for example. Such a printhead may be incorporated in a heat sensitive printer, thermal transfer printer, labelling printer, facsimile, word processor, typewriter, card printer, image printer, time recorder or the like.
For the convenience of description, reference is now made to FIGS. 20 through 22 showing a prior art thermal printhead.
The prior art printhead 3" shown in FIG. 20 is a serial-type printhead which comprises a head substrate 1" and a printing portion 2" provided on the head substrate 1". As shown in FIG. 21, the printing portion 2" is provided at a head glaze strip 4" formed on the head substrate 1" adjacent to one longitudinal edge thereof. The glaze strip 4" is formed to have a corner 11 which is relatively sharp for concentrating contact pressure relative to an article (e.g. paper) being printed.
As shown in FIGS. 21 and 22, a patterned resistor layer 5" is formed on the head substrate 1" and glaze strip 4", whereas a patterned conductor layer is formed on the resistor layer 5" to provide a common electrode 6" and a plurality of individual electrodes 7". The individual electrodes 7" are spaced from the common electrode 6" at the corner 11" of the glaze strip 4", so that the patterned resistor layer 5" provides an array of heating dots 8" along the corner 11" of the glaze strip 4". Further, a protective layer 24" of glass for example is formed on the patterned conductor layer 6", 7".
The thermal printhead having the above structure may be manufactured in the following manner.
First, as shown in FIG. 23, a master substrate 10" made of an insulating material is prepared. The master substrate 10" has plural rows (e.g. three rows) of unit head regions 9" which are subsequently separated from each other by cutting. Apparently, each of the unit head regions 9" corresponds to a head substrate (see the element 1" in FIG. 20). Further, the master substrate 10" is provided with a pair of positioning marks 14".
Then, as also shown in FIG. 23, head glaze strips 4" are formed on the master substrate 10" in the respective unit head regions 9" adjacent to and along their corresponding longitudinal edges. As a result, the glaze strips 4" in each row are longitudinally aligned.
The head glaze strips 4" may be formed by printing a glass paste with the use of a screen (not shown) and thereafter baking the glass paste for curing. Due to the surface tension of the glass paste, each of the strips 4" will have a generally arcuate outer surface, as shown in FIG. 24.
Then, as shown in FIG. 25a or 25b, the master substrate 10 is subjected to a corner forming step along each of the head glaze strips 4". In this step, an inclined cut surface 26 is formed to provide a corner 11" at the glaze strip 4".
Then, a uniform resistor layer (not shown) is formed on the master substrate 10" in each unit head region 9", and a uniform conductor layer (not shown) is formed on the resistor layer.
Then, the resistor layer together with the conductor layer is etched in a predetermined pattern (see FIGS. 21 and 22) by photolithography. In this step, a resist layer (not shown) is first formed on the conductor layer, and the resistor layer is patterned by using a photomask. The photomask has an alignment mark 12" (See FIG. 26) for alignment with each of the positioning marks 14" of the master substrate 10" (FIG. 23), thereby facilitating the mask alignment.
Finally, a protective layer 24" (see FIG. 21) is formed by a known step on the patterned resistor layer 5" and patterned conductor layer 6", 7" in each unit head region 9" (see FIG. 23), and the master substrate 10" is fully cut to separate the respective unit head regions 9".
According to the method described above, it is preferable to perform the corner forming step in a manner such that the formed corner 11" is located at the widthwise center of each glaze strip 4" or slightly offset inwardly from the widthwise center. However, it has been found that production errors are likely to occur at the time of performing the corner forming step if no countermeasure is taken. For instance, the glaze corner 11" may be positionally offset laterally outwardly from the widthwise center of the glaze strip 4", as shown in FIG. 25a. In this case. the glaze corner 11" is insufficiently sharp and therefore fails to provide an intended contact pressure concentration for improving the printing quality. Further, the volume of the glaze strip 4" may be excessively large, which also deteriorates the printing quality.
Conversely, it is also possible that the glaze corner 11" be offset excessively inwardly from the widthwise center of the glaze strip 4", as shown in FIG. 25b. In this case, the volume of the glaze strip 4" is too small to provide an intended heat retaining function, thereby failing to improve the printing quality. Further, the inclined cut surface 26" of the substrate 10" may be followed by an unwanted platform 13" which may come into damaging contact with an ink ribbon or printing paper.
Moreover, since the mask alignment for the etching or photolithography step is performed by referring to each positioning mark 14" but not the glaze corner 11" itself, the array of heating dots 8" formed by the etching step may positionally deviate from the glaze corner 11" if the glaze corner 11" itself deviates positionally in the preceding corner forming step. Apparently, such a positional deviation of the heating dots array 8" will also results in a deterioration of the printing quality.